1. Field of the Invention
This invention relates to electrochemical cells, and more particularly to novel electrodes and membranes for electrochemical cells.
2. Background
Our society has come to rely on electrochemical cells, such as batteries, fuel cells, and electrolytic cells, to perform a wide variety of functions. Batteries in particular are used to power a myriad of devices, including computers, cell phones, portable music players, lighting devices, as well as many other electronic components. Batteries are currently being developed to power automobiles and/or provide load-leveling capabilities for wind, solar, or other energy technologies. The “information age” increasingly demands portable energy sources that provide lighter weight, higher energy, longer discharge times, more “cycles”, and smaller customized designs. To achieve these advances, technologists continue to work to develop batteries with higher energy densities while still providing acceptable safety, power densities, cost, and other needed characteristics.
Batteries and other electrochemical cells come in a wide variety of different chemistries and structures. Each chemistry and/or structure has different advantages and disadvantages. For example, batteries that utilize dense ceramic membranes as the primary electrolyte advantageously have higher faradaic efficiencies (in some cases, close to 100 percent) and longer shelf lives (i.e., lower rates of self-discharge) than other battery chemistries. Nevertheless, batteries that utilize dense ceramic membranes as the primary electrolyte have their drawbacks. For example, these ceramic materials are often brittle, making them susceptible to breakage and catastrophic failure. These ceramic materials may also be poor ion conductors at room temperature.
For example, Nasicon is one type of ceramic material that is selective to sodium ions (i.e., conducts only sodium ions). However, the room temperature ionic conductivity of this material is only about 1 mS/cm, so that even modest Na flux densities require membranes less than 100 microns thick. A Nasicon membrane of this thickness is too fragile to be utilized in free-standing applications. Another challenge of using ceramic membranes in electrochemical cells such as batteries, fuel cells, or electrolytic cells is that of providing a good three-phase boundary exchange between a reactant (which may be gaseous or liquid or a reactant dissolved in a liquid), an electronically conductive phase, and an ionically conductive ceramic solid phase.
In view of the foregoing, what are needed are structures and methods for utilizing dense ionically conductive ceramic membranes, such as Nasicon or Lisicon, as the primary electrolytes in electrochemical cells, such as batteries, fuel cells, and electrolytic cells. Further needed are structures and methods to increase the mechanical robustness of such dense ceramic membranes. Yet further needed are structures and methods to provide a good three-phase boundary exchange between a reactant, an electronically conductive phase, and an ionically conductive ceramic solid phase.